From: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Subject: mm/shmem: do not wait for lock_page() in shmem_unused_huge_shrink()

shmem_unused_huge_shrink() gets called from reclaim path. Waiting for page lock may lead to deadlock there.

There was a bug report that may be attributed to this:

http://lkml.kernel.org/r/alpine.LRH.2.11.1801242349220.30642@mail.ewheeler.n...

Replace lock_page() with trylock_page() and skip the page if we failed to lock it. We will get to the page on the next scan.

We can test for the PageTransHuge() outside the page lock as we only need protection against splitting the page under us. Holding pin oni the page is enough for this.

Link: http://lkml.kernel.org/r/20180316210830.43738-1-kirill.shutemov@linux.intel.... Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure") Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Reported-by: Eric Wheeler linux-mm@lists.ewheeler.net Acked-by: Michal Hocko mhocko@suse.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Cc: Hugh Dickins hughd@google.com Cc: stable@vger.kernel.org [4.8+] Signed-off-by: Andrew Morton <> ---

mm/shmem.c | 31 ++++++++++++++++++++----------- 1 file changed, 20 insertions(+), 11 deletions(-)

diff -puN mm/shmem.c~mm-shmem-do-not-wait-for-lock_page-in-shmem_unused_huge_shrink mm/shmem.c --- a/mm/shmem.c~mm-shmem-do-not-wait-for-lock_page-in-shmem_unused_huge_shrink +++ a/mm/shmem.c @@ -493,36 +493,45 @@ next: info = list_entry(pos, struct shmem_inode_info, shrinklist); inode = &info->vfs_inode;

- if (nr_to_split && split >= nr_to_split) { - iput(inode); - continue; - } + if (nr_to_split && split >= nr_to_split) + goto leave;

- page = find_lock_page(inode->i_mapping, + page = find_get_page(inode->i_mapping, (inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT); if (!page) goto drop;

+ /* No huge page at the end of the file: nothing to split */ if (!PageTransHuge(page)) { - unlock_page(page); put_page(page); goto drop; }

+ /* + * Leave the inode on the list if we failed to lock + * the page at this time. + * + * Waiting for the lock may lead to deadlock in the + * reclaim path. + */ + if (!trylock_page(page)) { + put_page(page); + goto leave; + } + ret = split_huge_page(page); unlock_page(page); put_page(page);

- if (ret) { - /* split failed: leave it on the list */ - iput(inode); - continue; - } + /* If split failed leave the inode on the list */ + if (ret) + goto leave;

split++; drop: list_del_init(&info->shrinklist); removed++; +leave: iput(inode); }

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